1. Field of the Invention
This invention relates to a free fall sensor detecting a fall state.
2. Description of the Related Art
A free fall sensor has conventionally been incorporated in appliances such as portable computers to detect a fall state, thereby protecting hard disks built in the appliances against damage caused by shock due to the falling. Measures are taken on the basis of a detection signal generated by the free fall sensor. For example, a magnetic head of the hard disk is moved to an evacuation position.
The free fall sensors are required to be miniaturized in order to be built into appliances. JP-A-2000-195206 (hereinafter, “document 1”) discloses one example of the foregoing free fall sensors. The free fall sensor disclosed in document 1 comprises a cylindrical electrically conductive container serving as a fixed electrode, a flexible beam or bar-like spring horizontally cantilevered in the container and connected to one of two ends of an electrically conductive pin with the other end facing an exterior, and a steel ball provided as a weight at a free end side of the spring. The steel ball functions as a movable electrode which is brought into contact with and departed from the container.
In a normal stationary state of the free fall sensor of document 1, the spring is subjected to the gravity of the steel ball thereby to be flexed, whereby the steel ball is brought into contact with an inner face of the container such that an electrical path is formed. On the other hand, apparent gravitational acceleration acting on the steel ball is reduced during fall such that the steel ball is in zero gravity. As a result, the spring returns to the horizontal free state by its resilience, whereupon the steel ball parts from the inner face of the container, breaking the electrical path. Thus, the aforementioned free fall sensor is capable of detecting the fall state on the basis of break of the electrical path.
JP-A-2001-185012 (hereinafter, “document 2”) discloses another free fall sensor. The free fall sensor disclosed in document 2 employs a compression coil spring as the spring and has substantially the same function as the above-described free fall sensor of document 1. In particular, a cylindrical weight is provided around the compression coil spring, so that the length of the sensor can further be reduced, whereupon the sensor can further be miniaturized.
However, a single bar-like spring is employed as the spring in the free fall sensor of document 1. In a case where the weight flexes the cantilevered free end when the sensor has suffered shock due to fall, bending stress is concentrated on the bar-like spring such that the spring is liable to be plastically deformed partially or buckled. Accordingly, the spring serving as the movable electrode cannot be retained at a normal position thereof for a long time of use, thereby resulting in an instable switching operation. Consequently, the sensor of document 1 has problems in the durability of the spring. Moreover, when the weight of the steel ball or the spring force of the spring is reduced for the purpose of further miniaturization of the free fall sensor, other factors of instability such as poor contact are added since a contact pressure is reduced between the steel ball and the conductive container. In consideration of the flexibility and durability of the spring and spherical weight, it is difficult to design and manufacture a free fall sensor which is well balanced in these respects. Thus, the free fall sensor of document 1 is unsuitable for further miniaturization and particularly for reduction in the thickness which has recently been desired keenly.
On the other hand, the compression coil spring used in the construction of document 2 can reduce the spring force and act effectively to return to the free state reliably in the occurrence of fall. Consequently, the freedom in the design of coil spring and weight can be increased. Moreover, since the coil spring has sufficient elasticity, it is effective in preventing partial buckling thereof as in the aforementioned bar-like spring. Further, even when the length of the compression coil spring is rendered smaller as compared with that of the bar-like spring, a sufficient amount of flexure can be achieved.
The construction that the cylindrical weight is provided around the coil spring is effective for reduction in the length thereof. However, this construction has a definite limitation in reduction in the thickness or height. For example, the overall thickness (or diameter) of the free fall sensor has a limit of about 5 mm. Additionally, it is never easy to produce a small-sized weight with high decision by gold-plating the weight in order that a unique shape of weight may serve as an electrode. Further, the assembly of the free fall sensor is complicated. Furthermore, the coil spring is superior to the bar-like spring since it is hard to suffer the plastic deformation due to buckling. However, the weight is applied as shock to the coil spring during fall, and moreover, the direction of shock is biased. Accordingly, there is a possibility that part of the coil spring may suffer plastic deformation, which results in an instable operation of the coil spring or which is a problem in the durability.
Furthermore, the weight forcibly collides against the conductive container when each of the above-described free fall sensors suffers shock due to fall, whereupon an abnormal sound is produced. Additionally, a point of contact is concentrated on a single point. Consequently, there is a possibility that soil may be interposed between the electrodes or a long time service of the sensor may result in formation of an oxide film between the electrodes. In this case, a stable electrical path cannot be formed. In particular, when the size of the free fall sensor is further reduced, the spring force tends to be reduced and the contact pressure also tends to be reduced. Thus, each above-described free fall sensor has a problem of further disadvantage in the practical use.